Resistive random-access memory (RRAM) is a non-volatile memory type under development. RRAM has recently gained much interest as a potential replacement for FLASH memory.
The basic idea is that a dielectric, which is normally insulating, can be made to conduct through a filament or conduction path formed after application of a sufficiently high voltage. The conduction path formation can arise from different mechanisms, including defects, metal migration, etc. Once the filament is formed, it may be reset (broken, resulting in high resistance) or set (re-formed, resulting in lower resistance) by an appropriately applied voltage.
At the basis of RRAM is a metal-insulator-metal (MIM) stack. HfO2 has been of great interest as the insulator in the MIM stack. However, a better performance has been demonstrated by not only using HfO2 as the insulator but by using a bi-layer of a stoichiometric HfO2 layer and a non-stoichiometric O-deficient HfOx (x<2) layer. For HfO2, the commonly accepted mechanism of filament creation and destruction occurs by the diffusion of oxygen vacancies. Oxygen vacancies lead to defect states in the HfO2 dielectric; if a large number of oxygen vacancies are present (locally), the HfO2 dielectric becomes conductive. In such a bilayer stack, the non-stoichiometric O-deficient HfOx (x<2) layer can act as a sink for oxygen. So far, this O-deficient HfOx layer has always been deposited by PVD but for integration, it would be of great interest to deposit the layer by ALD, which is more manufacturing friendly and which is the method typically used for HfO2 deposition. A general description of ALD is disclosed in [0004] to [0009] of US2005/0227003. So far, an ALD process for HfOx with x<2 has been elusive since no stable phase other than HfO2 exists in the Hf—O phase diagram and the deposition of suboxides by ALD is generally difficult. There is therefore a need in the art for an alternative material which can be deposited by ALD.